A UBI scheme can't just create value from thin air; it has to come from either taxes or from devaluing the currency by printing more, which is basically a tax by another name.* Given that fact, there must be at least some class of relatively wealthy people who come out worse off, even after their UBI checks. The bigger the check, the larger that class of people. Since those people aren't idiots, they'll apply political counterpressure against raising the rate.

Because there's still the same total amount of money in the system, (inflation adjusted) prices can't go up too much. According to the Econ 101 model, goods with a fixed supply will probably get more expensive since there are more potential customers wanting the same amount of stuff, but goods with an extremely flexible supply will get cheaper, again, because there are more potential customers to spread the fixed costs over.

* There's also a sort-of-UBI variant where the funds come from monetizing some big nationalized resource and distributing the proceeds (e.g. the Alaska Permanent Fund Dividend). In that case the above doesn't apply, but neither does your fear. Since in that case the pool of money is dictated by the market value of the resource the voters can't directly impact how much they'll receive.